A cloning strategy was devised for construction of recombinants of cloned influenza NP DNA (coding for the nucleoprotein) and a bovine papilloma virus DNA vector. This NP-BPV recombinant DNA was used for transfection of a mouse cell line in order to establish persistent expression of influenza nucleoprotein. Recombinant DNA transfected cells were morphologically transformed and produced NP that was localized mainly to the nucleus during early passage of transfected cell cultures. Subcloning of transformants yielded a heterogeneous cell population and NP exhibited a more cytoplasmic distribution. These observations suggest that persistent expression of NP was accompanied by alteration of cell phenotype. When analyzed by sucrose gradient sedimentation the NP product was detected in the low molecular weight fractions suggesting that little or no encapsidation of RNA by NP had occurred. Full-length NP was immunoprecipitated with specific antiserum from cell lysates of transformants. This NP was phosphorylated similar to NP produced during influenza virus infection. Other techniques for establishing persistent expression of influenza NP DNA and other influenza cloned DNAs are under study. These include a simian cell system which is permissive for influenza virus infection so that complementation and gene rescue can be attempted. In these studies a vector that contains dihydrofolate reductase (DHFR) DNA as a selectable marker is being used for transfection and persistent expression.